This invention relates generally to a control system of the type wherein manual control devices such as a control stick are provided which have an electro simulated variable force versus displacement characteristic, referred to herein as variable rate feel. Hand controller systems having this electro simulated variable rate feel are commonly referred to as active hand controllers. The related invention referenced hereinbefore utilizes electric motors to produce desired hand controller force and position response. These systems require sensors which can provide accurate and stable information about the position of the hand grip and the force or torque exerted upon the hand grip by the operator. The present invention relates specifically to a force sensor or moment cell capable of providing force information as described in application Ser. No. 07/957,278, now U.S. Pat. No. 5,264,768. The present invention allows such a force sensor to be used in an environment such as an aircraft or spacecraft where significant acceleration forces may be encountered.
FIG. 1 illustrates a passive or spring centered hand controller 12 of the type found in the prior art including hand grip 14, shaft 16, gimbal 18, and position resolver 20, shaft 22, arm 23 and spring arrangement 24. In the single degree of freedom arrangement shown, gimbal 18 allows rotational motion of the hand controller 12 about an axis parallel to shaft 22. It is well known that if hand controller 12 is to be used in an environment where significant acceleration will be experienced, then a counterweight 26 must be provided. Counter weight 26 is typically a weight attached to gimbal 18 and sized to statically balance gimbal 18, including hand grip 14 and shaft 16 so that no net torque about shaft 22 will be generated by longitudinal acceleration.
In some active hand controller systems, the counterbalance approach just described may not be necessary due to the capability of the active feedback system to resist any unbalanced gimbal force. The elimination of gimbal counterweight 26, where this is possible, provides a weight savings that is especially advantageous in aircraft applications. There is a need, however, to balance inertial forces which the active hand controller does not inherently balance, for example, where the hand controller has an axis of rotation displaced from the gimbal axis of rotation, as is the case with the moment cell described hereinbefore. In the present design concepts for an active hand controller utilizing the moment cell and having two degrees of freedom, i.e., pitch and roll, there is no space available directly beneath the moment cell for a counterweight. Thus a need exists for a device to balance the inertial forces associated with the moment cell previously described.